Given that the whole premise of What If..? is to explore new universes of possibility within the Marvel Cinematic Universe, hinging on the consequences of single moments of diversion, it's not surprising that the show ended up revealing a lot of new information about the franchise's now-familiar setting.
Whether relating to the structure of the multiverse itself, the powers and potential of characters within its bounds, or else highlighting concerns that could be addressed in future MCU projects, the first season of What If..? gave viewers a lot to think about. The series contained a multitude of surprising revelations about characters fans thought they knew in their mainline, live-action MCU adventures.
Absolute Points In Time
Previously referred to as "nexus points" in Loki, absolute points in time are re-explored as a feature of the multiverse in "What If..? Doctor Strange Lost His Heart Instead Of His Hands," when Doctor Strange repeatedly tries to save Dr. Christine Palmer to no avail, because of her death being a fixed certainty in his universe.
Despite the valid criticisms of this storyline in regards to the fridging of Palmer to motivate Strange's actions, it's an important episode because it shows that nexus points aren't the same in every part of the multiverse - Christine survives this fate in the Doctor Strange movie. Additionally, the idea of nexus points suggests that one of Scarlet Witch's unexplored Avengers powers in the MCU, embodying a nexus being, may arise in future stories.
Thanos Was Capable Of Being Redeemed
Though his actions in the main MCU continuity were horrific and inexcusable, the second episode of What If..? ("What If..? T'Challa Became a Star-Lord") provides a vision of another kind of Thanos, one who was shown compassion and learned to take another path in life.
While there are many variants of Thanos in Marvel Comics, it's rare to see a version of the character that isn't forever tied to ideas of annihilation. Showing the grand influence that T'Challa's pragmatism can have on even the most unlikely of allies, this revelation begs the question, could the live-action Thanos have been affected in a similar way, or were the two's circumstances too far removed from one another?
Ultron's Full Powers Were Never Seen
One of the criticisms levied at Avengers: Age Of Ultron was the way that Ultron as a villain was handled, as the robot never seemed to come into his full power in the way that some more successful Marvel villains have. "What If..? Ultron Won" sets up the character as What If..?' season 1's main villain, showcasing far more of his potential.
In acquiring the use of Vision's body and subsequently the Infinity Stones, Ultron's megalomania reaches new levels of destruction, revealing that his desire to conquer stretched far further than just the Earth. This version of Ultron feels more in line with the Thanos of Infinity War, and further explores the repercussions of an AI gone rogue.
The Capabilities Of Scarlet Witch's Magic
A number of scenes in the MCU prove that Scarlet Witch is the strongest Avenger, but this is demonstrated even when she's in an extremely altered state during "What If..? Zombies?!," having been infected with the titular zombie virus that has devastated that timeline's Earth and almost wiped out humanity.
The nature of Scarlet Witch's chaos magic seems to mean that it transcends even a quantum virus, meaning that her powers can manifest even when her mind has been so drastically changed, showing that her skillset can act as a survival mechanism without much conscious input from her.
Collapsed Universes Can Be Preserved
After the lengths to which Doctor Strange goes to in order to save Christine Palmer leave him altered beyond recognition, with dark powers that bubble below the surface, the Watcher sees that the universe's disintegration is halted at its very end, leaving Strange in a small pocket of preserved life.
Viewed as a many-faced crystal sphere that parallels the shards through which the Watcher observes the multiverse's many realities, this moment of self-preservation proves important not only because Strange's powers are needed later on in the season, but also because it establishes a way that failing universes can be spared, which may prove important in upcoming MCU entries like Doctor Strange in the Multiverse of Madness.
The Watcher Can Be Observed
Only comic book fans know the extent of some of the Watcher's powers, but it came as a shock to many to see the Watcher himself observed in the series, first of all by Doctor Strange and later by Ultron wielding the Infinity Gauntlet.
Having been established as a passive force of the multiverse resigned to inaction, the perception of his existence by such influential characters shook up the character's commitment to not interfering. Furthermore, knowing that the Watcher can be seen by powerful individuals suggests he could one day appear in live-action MCU stories.
The Dangers Of The Quantum Realm
The origins of What If..?'s zombie outbreak comes about when an attempt to rescue Janet Van Dyne from the Quantum Realm is made, as first seen in Ant-Man and the Wasp. Since being lost to that place though, Janet had contracted the zombie virus and when she was rescued, it was already too late to save her.
Although the exact reason for the virus's presence in the Quantum Realm is unclear at this point, this reveal does provide more insight into the perils of the mysterious place, a danger which may well be unpacked further in the upcoming movie Ant-Man and the Wasp: Quantumania. Fans have now seen that this realm is capable of great good by holding the key to reversing Thanos' snap and evil as it also contained the virus that nearly wiped out another version of the MCU.
Arnim Zola's Full Skill Set
Whilst Arnim Zola's presence as a computer program had an impact on the storyline of Captain America: The Winter Soldier, his scenes were fairly brief and didn't showcase his full skill set - even after death - in the way that What If..? did.
The fortitude of Zola's programming in his ability to overpower an Ultron with the power of all the Infinity Stones is huge, and the revelation that multiple copies of him existed outside the one found by Natasha and Steve in New Jersey means that the character and his machinations could be seen once more.
The Multiverse Has Loki Variants Running Wild
Loki variants are a popular occurrence in both the comics and the show, where they were integral to the first season's plot, and often pruned by the TVA for disturbing the proper order of what was called the Sacred Timeline.
Given how the end of Loki's first season unleashed the full chaos of the multiverse, What If..? sees the appearance of Loki variants unrestrained by any interference, with those like frost-giant Loki in "What If..? Thor Were an Only Child" showing just one path of possibility for the trickster. His presence here sets up a whole new range of prospects for Loki's second season.
Steve Rogers Doesn't Need The Super Soldier Serum To Be A Hero
It may seem obvious in hindsight, given that Dr. Erskine always stated that his Super Soldier serum only enhanced the aspects of a person's character already most apparent within them, but seeing Steve Rogers make his debut as the Hydra Stomper in What If..?'s first episode proved he was always destined to be a hero.
Even without the serum or a shield, Steve's motivations remained remarkably true to form. Additionally, this episode showed more parallels between Tony Stark and his father Howard, given the similarity of Steve's new tech and early versions of the main-universe Iron Man suit. In both universes, the Stark men have a big influence on Steve's heroic journey.